1. Field of the Invention
The invention relates to a method and apparatus for detecting the residual quantity of toner in an image forming device, and particularly to a method and apparatus for detecting the amount of toner in image forming devices such as electrophotographic printers and copy machines in which the toner is mixed by a mixing member during the printing operation.
2. Description of the Related Art
Generally, in image forming devices such as electrophotographic printers, copy machines and fax machines, an electrostatic latent image corresponding to an image to be printed or copied is optically formed on a photoconductor drum. The latent image is then developed with a toner into a toner image, which is transferred to and fixed on a recording sheet to complete the printing or copying operation. As the printing or copying operation is repeated, the toner is gradually consumed. When the toner is reduced below certain level, the printed or copied image becomes thinned out so as to provide an unclear printed or copied image. It is usual, therefore, to detect the residual quantity or density of the toner using a toner sensor disposed at a toner mixing chamber wherein a mixing member is turned for mixing and frictionally charging the toner. The toner sensor detects the residual quantity or density of the toner mixture and provides an output voltage in accordance with the amount of the toner.
Usually, the image forming device is equipped with at least one toner indicator for indicating a need for replenishment of the toner or replacement of a toner container. When the quantity or density of the toner drops below a specified value, the toner indicator is actuated to inform the user to replenish the toner or replace the toner container.
FIG. 1 is a cross-sectional view of a conventional developing unit of the sort often employed for electrophotographic printers etc. As seen in FIG. 1, the unit includes a developing unit 1, and a photoconductor drum 2. The developing unit 1 has a mixing chamber 10 where the toner 9 is mixed and charged by friction, a toner separating portion 20, and a toner sensor 30.
A toner mixing member 11 is mounted in chamber 10 for stirring and frictionally charging the toner 9. The toner 9 is fed to a magnet roll 21 of the toner separating portion 20. As magnet roll 21 is rotated, the toner 9 is carried on the surface thereof. The thickness of the toner on the roll 21 is regulated by a doctor blade 22. The toner then comes into contact with the surface of the photoconductor drum 2 facing the magnet roll. A bias voltage is applied to the magnet roll 21 and the toner is transferred onto a electrostatic latent image formed on the surface of the photoconductor drum to thereby form a toner image according to the difference between the bias voltage and the surface potential of drum 2.
FIG. 2 is a perspective view illustrating the mixing member 11 of FIG. 1. Mixing member 11 includes a rotational shaft 11c which carries four arms 11a. Two of the arms 11a are mounted on the same side of the shaft 11c and the other two arms 11a are mounted on the opposite side thereof. The free ends of the arms 11a are connected by two bars 11b.
As shown in FIG. 3, toner sensor 30 is attached to the toner container 12 so as to detect the residual quantity or density of the toner in chamber 10. As shown in FIG. 4 toner sensor 30 comprises a differential transformer having a drive coil L1, a reference coil L2, and a detection coil L3. These coils L1, L2 and L3 are wound around the same core 31. A high-frequency signal of 500 KHz is applied to the drive coil L1 from an oscillator OSC.
There are two types of developers for image forming device. One type is a single component developer consisting only of the toner, and the other types is a two-component developer which contains the tone and a magnetic carrier such as ferrite or iron. Recently, a new type of two-component developer has become known, wherein the rate of usage of the carrier is very small as compared with the rate of usage of the toner. This new type of two-component developer is sometimes referred to as a 1.5 component developer.
When a two-component developer which is a mixture of magnetic carrier and the nonmagnetic toner is used, when the relative amount of the toner is high in a given volume, the relative amount of the magnetic carrier substances to too low to cause an increase in the magnetic resistance of the developer. On the other hand, if the relative amount of the toner becomes lower in the same volume, the relative amount of the carrier increases so as to reduce the magnetic resistance. The output voltage of the detection coil L3 changes in response to the relative amount (density) of the toner in the mixture, and the output voltage Vo of the toner sensor changes accordingly. Thus, the density of the toner is detachable according to the output voltage Vo of the toner sensor 30.
When the 1.5 component developer, which is a mixture of a small quantity of magnetic carrier and a large quantity of the nonmagnetic toner is used, the toner sensor 30 cannot detect the density of the toner. However, as the toner is consumed, the magnetic resistance of the developer changes depending on whether the developer is above, below, or around the surface of the toner sensor. Accordingly, the residual quantity of toner in the chamber 10 is detectable according to the output Vo of the toner sensor 30.
While the toner sensor 30 is detecting the residual quantity of the toner 9, the toner 9 is being stirred and moved by the mixing member 11. The output voltage Vo of the toner sensor 30, therefore, oscillates as shown in FIG. 5 as the mixing member 11 rotates. As shown in FIG. 5, the mixing member 11 starts to rotate at time t1, the rotational speed thereof becomes constant after time t2, and the printing operation of the image forming device is carried out between time t2 and t3. The rotational speed of the mixing member 11 decreases after time t3, and the mixing member 11 stops at time t4.
The amplitude of the output voltage Vo of the toner sensor 30 as a function of the acceleration or deceleration of the rotation of the mixing member 11. When the mixing member 11 ceases to rotate, the output voltage Vo of the toner sensor 30 indicates a high or low value. In a case where the mixing member 11 stops moving at a point where a large quantity of the toner 9 is disposed on the toner sensor 30, the output voltage Vo of the toner sensor 30 will be high. This condition is indicated by dot and dashed lines A in FIG. 3. If the mixing member 11 should stop just after passing over the toner sensor 30, the output voltage Vo of the toner sensor 30 will be low because the quantity of the toner 9 on the toner sensor 30 will have been reduced by the mixing member 11. This condition is indicated by the phantom lines B in FIG. 3.
In this way, the relationship between the toner 9 and the toner sensor 30 changes according to the rotational position of the mixing member 11. In conventional devices, the conditions described destabilize the output voltage of the toner sensor 30 and cause an incorrect detection of the residual quantity of the toner.
When detecting the density of the toner 9, the output voltage Vo of the toner sensor 30 also fluctuates depending upon the rotation of the mixing member 11. Thus, the output voltage Vo becomes larger or smaller depending on the stopping position of the mixing member 11, and therefore, the density of the toner 9 is not correctly detected.